In the electrolytic production of magnesium, MgCl.sub.2 is decomposed into liquid magnesium and chlorine in a fused salt electrolysis cell according to the following equation: EQU MgCl.sub.2 =&gt;Mg.sub.1 +Cl.sub.2(g)
Conventionally, the electrolyte comprises MgCl.sub.2, NaCl, CaCl.sub.2 and other minor alkali & alkali earth chlorides which are well known in the art. A major problem associated with the magnesium chloride electrolyte is the presence of magnesium oxide (MgO), which is highly detrimental to the efficient operation of the electrolysis cell. For example,
MgO migrates towards the cathode and coats it with a thin layer that has the effect of creating additional resistance to electrical conductivity and increases power consumption of the cell; PA1 the thin layer of MgO on the cathode also renders the latter less wettable, causing the formation of fine droplets of magnesium that are not easily recoverable from the electrolyte; PA1 the fine droplets of magnesium may then become coated with an oxide film and have their densities increased to a point where they are dragged into the sludge at the bottom of the cell. Further, the droplets may also prevent coalescence with other magnesium droplets and therefore never gain enough buoyancy force to be collected at the top of the cell. In either case, the consequence is that magnesium is lost; PA1 MgO settles and pulls electrolyte along with it to form a cement-like formation at the bottom of the cell, resulting in the necessity to frequently rebuild the cell, a costly procedure in terms of time and production lost; and PA1 MgO reacts with the graphite at the anode to produce carbon dioxide and magnesium chloride, thus increasing the anode to cathode distance and causing voltage drop, thus resulting in a significant decrease in the life of the cell. PA1 a furnace containing the molten material; PA1 a sealed recipient for receiving filtered molten material, the recipient being coupled to the furnace with a pipe having one end submerged in the molten material in the furnace and comprising a syphon provided with a filter, and the other end in the recipient; and PA1 a pump coupled to the recipient to remove air therefrom and maintain a vacuum, whereby upon starting the pump, the molten material is drawn from the furnace to the recipient through the filter in the pipe, and the filtered molten material is recovered in the recipient. PA1 continuously feeding material to a furnace to melt the material, the furnace being coupled to a sealed recipient with a pipe having one end submerged in molten material in the furnace and comprising a syphon provided with a filter, the other end of the pipe being in the recipient, the recipient having a pump coupled thereto to remove air and maintain a vacuum; PA1 starting the pump to create and maintain a vacuum in the recipient, thus drawing the molten material from the furnace into the recipient through the filter and the pipe; and PA1 recovering filtered molten material.
The presence of other oxides like sulphates, which are only slightly soluble in electrolytes, also presents significant problems, since they greatly decrease the current efficiency, even in quantities as low as a few hundredths of one percent. Although the mechanisms are not well understood, it is believed that a magnesium sulphide layer may be formed on the surface of the cathode, thus causing reduced current efficiency. Moreover, the sulphate affects the surface chemistry of the salt in such a manner that a stable foam is produced above the electrolyte which tends to trap magnesium therein.
As most magnesium electrolytic cell feeds are derived from an aqueous chloride solution subsequently dehydrated to produce magnesium chloride, the presence of MgO, sulphates and H.sub.2 O in the feed of electrolysis cells is a universal concern within the industry.
The presence of MgO is unfortunately almost unavoidable due to the thermodynamic equilibria existing in the cell. Further, water reacts with MgCl.sub.2 to form MgO, which significantly aggravates the problem. In typical magnesium plants, there is generally a unit operation to eliminate the moisture and the MgO present in the feed material.
Several methods exist to eliminate water and magnesium oxide. Examples of these are as follows:
1) In U.S. Pat. No. 3,742,199, MgCl.sub.2 prills (MgCl.sub.2.xH.sub.2 O - .about.2 wt % MgO) produced in a fluid bed dryer are contacted with huge quantities of HCl gas in a dehydration fluid bed tower. This process drives off the moisture, prevents hydrolysis and formation of more MgO.
2) The Oriana smelter in Ukraine, and Avisma and SMZ smelters in Russia use a carbochlorination process which contacts melted hydrated MgCl.sub.2 with carbon and chlorine in a shaft furnace. The reaction is between the MgO, water, the carbon and the chlorine to produce carbon dioxide, HCl and MgCl.sub.2 (see Kh. L., Strelets, "The chemistry and electrochemistry of magnesium production", translated by J. Schmora, Keter, Jerusalem, 1977 (also available as TT 7650003, U.S. Dept. Commerce, NTIS Springfield, Va., pp. 43-46).
3) Another known process, which is similar to the carbochlorination process, consist in contacting CO+Cl.sub.2 with melted hydrated MgCl.sub.2 in an agitated furnace. The mixture reacts with water and MgO to produce CO.sub.2 and MgCl.sub.2 and HCl. U.S. Pat. No. 4,800,003 discloses such process. In both methods discussed in paragraphs 1) and 2), as well as in this method, a large quantity of the water must react with reagents, thus slowing the kinetics and increasing the quantity of reagents required.
4) U.S. Pat. No. 5,565,080 uses a more efficient and sophisticated process in which no reducing agent is required and HCl contacts prills dissolved in electrolyte. The primary advantage of this method is that unlike the previous ones, it occurs at significantly lower temperatures (650.degree. C. vs 750.degree. C. or more); and the reagent only needs to react with the MgO fed to the chlorinator. Therefore, no magnesium chloride hydrolysis occurs because the thermodynamic driving force for hydrolysis is eliminated. Due to the above mentioned facts, the kinetics of this process are generally faster than most other processes.
All the methods mentioned above are chemical methods which involve injection of large volumes of reagent gases into fused chloride salts to prevent the formation of MgO and to reduce any MgO formed to MgCl.sub.2. One of ordinary skill in the art can appreciate the level of engineering and materials selection complexity associated with such operations. In addition, the capital/operating expenses and the safety concerns related to supporting the above mentioned technologies can be quite prohibitive in terms of implementation, not to mentioned the potential environmental effect that a leak of HCl or chlorine gas would have.
Russian plants have been known to use an alternative physical method to separate solid MgO particles from fused salt baths (see Strelets, Kh. L. "The Chemistry and Electrochemistry of Magnesium Production" Translated by J. Schmorak, Keter, Jerusalem, 1977. Also available as TT 7650003, U.S. Dept. Commerce, NTIS Springfield Va., p. 131-143). The technology entails settling of MgO in a carnalite containing furnace. Since their electrolysis cells are monopolar, and thus, much more forgiving in terms of acceptable levels of MgO in the feed because the anode and the cathode are relatively apart from one another, this process is fairly successful. It involves allowing the feed to have a long retention time in a holding furnace. The longer the retention time, the greater MgO particles may settle. Depending on the particle size distribution of the oxide, the lowest MgO concentration available from this type of process is in the order of 0.2-0.5 wt %. Therefore, such electrolyte cannot be considered suitable for use with modem sophisticated multipolar magnesium electrolysis cells with high efficiency and throughput, because such cells generally require MgO level lower than 0.1 wt %, and most preferably lower than 0.05 wt %.
In the aluminum industry, gravity filtration for removal of large solid particles in molten aluminum is common practice. Typically, large pore ceramic foam filters are used for such filtration, as described for example by Mills et al. in Light Metals, 1994, 1001-1005. A number of studies have also been done with the use of other media such as ceramically bonded crushed alumina, high temperature fabric screens and monolithic extrusions (Apelian et al. in Light Metals, 1981, 735-750. The filtration technology can be easily applied to molten aluminum, since the size of the particles present therein is generally greater than 20 .mu.m.
Die casting or gravity casting of metal components with less than 50 lbs. weight requires batch injection/filling of the molten metal into a preformed mold. Despite persistent efforts to prevent the formation of metal oxides, the latter still enter in the cast product during the casting step. Each time a fixed quantity of molten metal is ladled from the holding furnace into a mold, the freshly formed layer of metal oxides at the surface of the melt is disrupted and some metal oxides are introduced in the ladle.
Another problem in pressure/counter-pressure die casting operations involves the refinement of the molten metals from hydrogen and inclusions before casting parts therefrom. The thus treated metal is then pressurized to pump a predetermined volume from the holding vessel into a mold. Once the reservoir of molten metal in the holding vessel is depleted, the vacuum/pressure seal is broken and the empty vessel is replaced with a new vessel loaded with treated molten metal before casting operations are resumed. Such replacement of vessels requires approximately 10-20 minutes and penalizes the throughput of the casting equipment.
There is therefore a great need to develop a physical method to remove solid particles such as magnesium oxide from molten materials like magnesium chloride electrolyte or magnesium. Such method would be helpful in magnesium electrolysis by providing cleaner electrolyte. Further, the method could be advantageous if it may be coupled to die casting operations so that such operations may be conducted in a continuous manner without having to replace any empty vessels.